A conventional receiver may include at least one mixer to downconvert the frequency of an incoming signal. More specifically, the mixer typically multiplies the incoming wireless signal with a local oscillator signal to produce a signal that has spectral energy that is distributed at sums and differences of the local oscillator and incoming signal's frequencies. For a downconversion mixer, the desired output is the difference between the local oscillator and incoming signal frequency. If the local oscillator signal is a pure sinusoid only the spectral energy of the incoming signal that is at an intermediate frequency (IF) away from the local oscillator (LO) signal appears at the output of the downconversion mixer. However, for certain mixing applications, the local oscillator signal may be a non-sinusoidal, such as a square wave signal, which contains spectral energy that is located at a fundamental frequency and additional spectral energy that is located at harmonic frequencies of the fundamental frequency. Mixing the incoming signal with such a local oscillator signal causes the spectral energy of the incoming signal at IF away from the harmonics of the LO signal to also appear along with the desired signal at the downconversion mixer's output.
Harmonic rejection mixers exist that include multiple mixers such as Gilbert cell type mixers to each receive a scaled version of an incoming signal, where the outputs of each mixer stage are summed to provide a downconverted (or upconverted) output. Each mixer may operate at a phase difference from the other mixers, and each scaling factor that scales the incoming signal may be in accordance with a predetermined sinusoidal function such that the harmonic rejection mixer ideally rejects all harmonics except M×N +/−1, where M is any integer and N is the number of individual mixer stages. However, actual implementations do not operate according to this ideal. Instead, in practical implementations in a semiconductor integrated circuit (IC) process, various problems exist. These problems include difficulties in device matching among the different mixers, as random device mismatches between active devices, i.e., transistors in the mixers may cause the scaling factors to deviate from an ideal value, causing degradation in harmonic rejection. Furthermore, the phases of a LO signal provided to each branch may also deviate, causing harmonic rejection degradation.
To overcome such problems in conventional harmonic rejection mixers, very large device sizes are needed, which creates circuits that are very large and consume significant power. Furthermore, even if a large size is implemented such that the standard deviation of random mismatches is reduced (in turn raising power and area by a factor of 4), harmonic rejection can still be affected by the duty cycle of each LO waveform. Accordingly, positive and negative LO signals should be exactly 180 degrees out of phase, requiring additional well-matched components and operation at higher frequencies, again causing more consumption of power to achieve a desired performance level. In many designs, the amount of harmonic rejection that can realistically be achieved in such a mixer may be between approximately 30-40 dB, when operating at an LO frequency of several hundred MHz. Such performance may be acceptable for some applications. However, operation at this level can cause stricter tolerances for other components in a total budget for a given receiver design.
Thus, there exists a continuing need for a mixer that rejects harmonic frequencies that may be introduced by a local oscillator signal that is not a pure sinusoid.